The electrolytic double layer capacitor is constituted by constituting a polarizable electrode unit by an activated charcoal layer 1 and a collector electrode 2 for that layer, and by putting a separator 3 soaked with an electrolyte between a pair of polarizable electrodes.
Hitherto, as the constitution examples of electrolytic double layer capacitors of this kind, there have been the following two kinds. That is, the first one that is, as shown in FIG. 1, expanded metal or punching metal worked from aluminum sheet is used as collector body, a paste containing activated charcoal powder as main component and a binder of fluorine resin or the like as a polarizable electrode 5 is coated to be held on a surface of the collectors 4 by rolling rollers 7, and a pair of the collectors 4 and the polarizable electrode 5 with a separator 8 inbetween are wound up and electrolyte is injected thereto. FIG. 4 is a sectional structure of the electric double layer capacitor of this system, FIG. 5 is an enlarged view of a part of FIG. 4. 9, 10 are electrode leads, 11 is an outside face, 12 is a rubber sealing cap.
The second one has a structure as shown in FIG. 6, that is, conductive elastomer 13, insulative ring 14, separator 15 and a viscous mixture 16 of activated charcoal powder and an electrolyte are combined as shown in the drawing, and has a disk type outside view as shown in FIG. 7.
Both of these conventional designs use powder type activated charcoal as polarizable electrodes. Therefore they have many problems on characteristics and manufacturing.
In the one of the first type, the bonding force between the metal collectors 4 and the polarizable electrode 5 of activated charcoal is weak, and such tendencies are observed that the polarizable electrodes 5 drop off and peal off the collectors 4 or the bonding force between both ones becomes weak during use because of strain by the winding up and thereby internal resistance of the electric double layer capacitor gradually increases, to gradually decrease the capacitance. Furthermore, when the polarizable electrode 5 consisting of the activated charcoal powder and the binder are rolled onto the collectors 4, its coating efficiency is bad, and scattering of capacitance due to nonuniform coating and rolling of the polarizable electrodes 5 is given as the point to be improved. Furthermore, in conjunction with the structure, that which fulfils needs of known microelectronics, for instance of flat plate type structure, has been difficult to realize.
In order to resolve these structural defects, various proposals, for instance, raising contact strength of the polarizable electrodes 5 by roughening by blasting or the like operation, the surface of the collectors 4, or by strengthening the bonding force of the activated charcoal by adding a binder such as methyl cellulose to the activated charcoal powder, have been tried, but neither one has satisfactory points yet, though slight improvement of characteristics are observed.
Furthermore, in the one of the second type, though a flat plate type ones are obtainable enabling to respond to minimization of the appratus, there is a complicated point in working in the manufacture since powder type activated charcoal is used, and furthermore it is not satisfactory also from the view point of capacitance per volume performance.
Presently, though as the electric double layer capacitor of the second constitution the ones using an aqueous solution of sulfuric acid as the electrolyte are sold in the market, breakdown voltage per unit cell becomes 0.8 V in this type, and it does not fulfil a demand of high breakdown voltage.
Furthermore, as a modified example of the first type, an electric double layer capacitor wherein two sheets of carbon fiber are used as polarizable electrodes, and they are wound up in a roll type with a separator inbetween, and about 1 mm step is made on the opposing electrode only at the edge face and metal layer is formed by a spray method on both end faces to form collectors and lead terminals simultaneously, is devised. In the capacitor of this type, though forming of the end face electrodes become easy, the fundamental structure is the same as the conventional first type, and obtaining of further small type capacitor structure is difficult.